1. Field of the Invention
The present invention relates to a process for producing high impact polystyrene (hereinafter referred to as HIPS).
2. Description of the Prior Art
In recent years large amounts of various synthetic resins are used in vehicles such as automobiles and in many other fields. However, there are strong requests for improvements of these resins in mechanical properties as well as appearance properties including surface gloss. Specially in the case of HIPS, it is requested to improve impact resistance and visual appeal such as surface gloss at the same time.
To improve the impact resistance of polystyrene, a method of graft polymerizing styrene or a mixture of styrene and another monomer copolymerizable therewith on a butadiene rubber has been developed and is generally practiced in the industry.
In this case, however, the use of a high cis content polybutadiene (so-called high Cis-BR) commercially avilable in usual has posed problems such that not only the impact strength improving effect is insufficient but also a coloration of the HIPS occasionally takes place or the impact strength is rather lowered by the incident formation of gel particles. On the other hand, the use of a commercial polybutadiene of relatively high trans content (so-called low Cis-BR), produced in the presence of a lithium-based polymerization initiator, results in a HIPS of relatively low gel content and fairly improved impact strength. But also in this case, it is necessary for further improving the impact strength to increase the amount of the polybutadiene used or raise the average molecular weight of the polybutadiene. This makes worse the gloss of the HIPS and lowers the surface smoothness thereof. Thus simultaneous improvements of the impact strength and the visual appeal have not been accomplished in any case.